Was it built from the beginning with joists that can support a live load, or was it only designed for storage? There IS a difference. What is the interior height? Code requires a 7' height to be considered livable space and you have to allow for room for insulation. What type of access does it currently have? You will have to take up room for the stairs both in the attic and on the floor below. Does it currently have any windows? Are they legal egress size? I already mentioned insulation, but it's really important, as is how you are planning to heat and cool the additional space. A mini split would be the easiest, but if the system in your home is old, you may want to have new calculations done, add some ducting, and replace the home's current HVAC with a unit that can handle all of the space. If the home was built from the beginning to be convertible, and has a nice high pitch with around 9' interior height, then you could have an easy project. That's not the same thing as an inexpensive project. Just a less expensive project than a difficult to convert space would be. 50K is about average.

@T Burns I don't have an ATMOX system, but from what I've seen they have one inside and one outside sensor input on their controls. One outdoor should be sufficient, but for a large crawlspace I'd prefer more than one just because humidity CAN be localized if there is bulk water such as a flood or plumbing leak. My home had mold and rot only in certain areas--generally the east (downwind) side. The ATMOX optional louvers don't appear air-tight but passively flap down over the opening when the fan isn't running, like on a dryer vent. It's probably no worse than a typical sliding crawlspace vent. Plotting temp and RH over time is a great idea. From there you can obtain dew point and get an idea how often an ATMOX system would be able to run with just fans to keep the RH below 60%. With tight clearance between floor framing and the ground, the joists will track ground temperature by radiant heat exchange unless you have lots of airflow. You can get an idea of what RH will be at the joist surfaces by looking at those temperatures and figure out what max average dew point should be allowed. From there you can look at historical dew points for your area and get an idea of what percentage of the time fans can actually dry the crawlspace. As the ATMOX folks point out in their literature, it takes an extended problem to elevate wood moisture content enough to cause mold. You may be able to get by with a dehumidifier for half the year or less. But I would definitely not install one of these systems without a dehumidifier as backup, integrated into the control logic. If nothing else, it will come in handy if you get a flood or a fan goes bad and you need brute force drying. There might be some aspects of encapsulation that could benefit your situation, but I'm leery about arbitrarily introducing spray foam, foam board, or plastic-wrapped batts, like the Youtube encapsulation guys do, without really thinking through the moisture transport situation. You could easily end up trapping moisture or hiding rot or termite activity in an old house that wasn't built to modern codes. For example, my home has poly flashing between the rim & band joists and the brick, lacks a brick ledge, and was not built with pressure treated sills. If the brick really gets soaked, the framing gets damp. This is a once-a-year event at most, but it has a way to dry. The last thing I'd want to do is to trap moisture in the lumber. Also, my home's crawlspace vents also admit air to the gap between the brick and sheathing, and blocking those gaps with foam board will reduce ventilation behind the brick. Retrofitting an older home has to be taken case-by-case. I wouldn't trust most one-size-fits-all encapsulation contractors to consider that. I would probably never be totally satisfied with an ATMOX system, since the control logic isn't arbitrarily customizable, but it's an incremental improvement over full-time passive ventilation that we know doesn't work in summer. Automating the ventilation and combining occasional dehumidifier use may be all you need to banish the mold if there are no drainage issues. Thanks for the tip about the upcoming revision. I'll have to get in touch with them to see if they're adding fan diagnostics.

I have a crawl space. Vents should be open all the time, except for winter months (in cold areas) Air circulation is important. A good thick plastic sheeting should be properly applied to the dirt floor to keep moisture from underneath out of the crawl space. No insualtion under the house itself, unless you have done other special work to keep it a conditioned space. Since up until several years ago, we had never had a crawl space, we thought about insulating the underside of the house, but, were told no, not a good idea by every insulation company we contacted. This is the second house with a crawl space we have owned now. It stays dry as a bone underneath, but, we also have proper drainage outside too.

an add on in the back (and front??) came in at 200K ?? jesus. Is that with changing current roof to accommodate new pitch and a new basement w/that? you could tear down that house to the studs and build it back up for less than that. what was the sq ft for the addition? why can't you bump out the back from bedroom/guest bath and make a new master suite? you could also enclose the screened in porch. (if you add on from the kitchen would you move that back as well? ) or, if you bump out the entire back end, you could move the kitchen back and increase your current family space, plus you gain a master and extra bath seems to be enclosing the garage would give you the same space as adding on to the back bedroom/bath.